1. Field of the Invention
This invention relates to imaging in a scanning system and more specifically to bidirectional stage travel in a serpentine pattern in a raster scanning imaging system, and a method for correction for chevron error which occurs in such systems, as well as an auto focus mechanism for such systems.
2. Description of the Prior Art
Raster scanning is well known and is used e.g. for imaging on a television screen. In raster scanning a beam is scanned horizontally across the surface of the medium (the TV screen) to be imaged. The beam is typically turned on and off in order to define pixels of an image or absence thereof at any particular point. At the end of each scan line the beam is returned to the beginning of the next line without scanning so all "writing" (imaging) is in one scan direction.
For applications such as television, at the end of each scan line the beam is translated vertically (orthogonal to the fast horizontal scan direction) by a small amount in order to reach the next scan line.
In other applications (laser beam scanning) it is not easy to vertically translate the beam itself, so instead the medium is moved vertically, i.e. in the direction orthogonal to the laser beam scan direction (the fast scan direction). This is illustrated in present FIG. 1 where the medium is supported on a movable platform (a stage) which is not shown and the X and Y axis respectively indicate the scan direction, in this case of a laser beam, and the stage travel direction. This is called unidirectional printing since the stage only moves in one direction. The dotted lines indicate the return path at the end of each scan line. Thus in unidirectional printing the laser (or other beam) scans in what is defined as the X horizontal direction and the substrate is moved on its stage in one of the perpendicular vertical (Y axis) directions, for instance the +Y direction.
In the case where the stage travels in small steps taking a time less than the "fly back" time needed for the laser beam return path (dotted line) at the end of each scan line, it is of course trivial for the scan lines to be perfectly located at an angle 90.degree. to the direction of stage motion. However in another case where the stage travels at constant speed in the Y direction while the beam scans orthogonally to the stage travel direction, then the scan lines will exhibit a small angle error .gamma. as shown in FIG. 1, where the direction of the scan is slightly downwards and to the right. This is referred to as an angle error .gamma.. Regarding the scan lines in the right portion of FIG. 1, these indicate the next scan field which is undertaken as a separate scan path; however, the features written in this next scan field are intended to be continuous with the left scan field.
In case of unidirectional printing, at the end of each stage travel path, the stage must be returned back to its starting position, side stepped to the left by distance L (thus bringing the right portion of FIG. 1 to the left for illumination by the scanning laser beam or laser beams), before starting the next travel path in the same y direction again. In FIG. 1, L is the length of one scan; .DELTA.T is the time per scan line; if v is stage velocity, then .gamma.=d/L=.DELTA.Tv/L.
The angle error which occurs with the unidirectional printing of FIG. 1 is a constant value .gamma. for the entire image area and is easily overcome by a small rotation in the laser scan line relative to the medium. This is easily accomplished so as long as the stage travel velocity is a constant (constant both in magnitude and in direction). However, a more complex situation occurs (as illustrated in FIG. 2) where the stage travel is bidirectional or nonconstant. In this bidirectional printing, at the end of each scan field, rather than the stage returning to the bottom of the next (adjacent) scan field, the stage begins to translate in the opposite direction (from top to bottom, i.e. the -Y direction) for the next scan field after side-stepping a distance L.
FIG. 2 therefore shows the situation where in the left field the stage direction of movement is upwards, i.e. along the +y axis going away from the origin, whereas for the adjacent scan field shown in the right portion of FIG. 2, the direction of stage travel is in the opposite direction. Under these circumstances the angle error changes polarity for adjacent stage travel paths, or raster imaging fields. It cannot be eliminated at all times with simple rotation of the scan direction of the laser beams. Since such bidirectional stage travel is highly desirable in a laser scanning system in order to increase throughout, i.e. to reduce stage travel time, then chevron or herringbone shaped artifacts occur in adjacent fields. This can be seen with reference to FIG. 2 by understanding that each line ideally is a straight line continuously over two scan fields as shown in FIG. 1. However, in the case of FIG. 2 it can be seen each line that crosses the raster imaging field border is actually a shallow "V" rather than the desired straight line, due to the junction between two adjacent but oppositely scanned fields. A group of these shallow V's forms a so-called chevron pattern or herringbone pattern which is an undesirable artifact due to the angle error .gamma..
Another problem encountered sometimes in raster scanning is focussing of the laser beam on the medium. The medium, e.g. a printed circuit board substrate or a flat panel display substrate, may not have a perfectly flat surface due to manufacturing irregularities. These uneven portions of the surface may cause defocussing of the incident laser beam, thus reducing image quality. It would be useful to be able to overcome this focus problem also.